The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Spinal fixation devices are used in orthopedic surgery to align and/or fix a desired relationship between adjacent vertebral bodies. Such devices typically include a spinal fixation element, such as a relatively rigid fixation rod, that is coupled to adjacent vertebrae by attaching the element to various anchoring devices, such as hooks, bolts, wires, or screws. The fixation rods can have a predetermined contour that has been designed according to the properties of the target implantation site, and once installed, the instrument holds the vertebrae in a desired spatial relationship, either until desired healing or spinal fusion has taken place, or for some longer period of time.
Spinal fixation devices can be anchored to specific portions of the vertebra. Since each vertebra varies in shape and size, a variety of anchoring devices have been developed to facilitate engagement of a particular portion of the bone. Pedicle screw assemblies, for example, have a shape and size that is configured to engage pedicle bone. Such screws typically include a bone screw with a threaded shank that is adapted to be threaded into a vertebra, and a rod receiving head, usually including a U-shaped channel. The shank and rod receiving head can be provided as a mono-axial screw, whereby the rod receiving head is fixed with respect to the shank, or a poly-axial screw, whereby the rod receiving element has free angular movement with respect to the shank. In use, the shank portion of each screw is threaded into a vertebra, and once properly positioned, a fixation rod is seated into the channel. The rod is then locked in place by tightening a set-screw, plug, or similar type of fastening mechanism into the rod receiving head.
While current spinal fixation systems have proven effective, it can be difficult to position rods within the rod receiving head of various fixation devices prior to locking the rod with the fastening mechanism. In particular, it can be difficult to align and seat a rod into the rod receiving head of a poly-axial implant since the rod receiving head has poly-axial freedom of movement with respect to the shank. More particularly, the poly-axial freedom of movement of the rod receiving head can allow the receiving head to “flop,” thereby requiring the surgeon or an assistant to hold the receiving head in the desired position during rod introduction.
Accordingly, there remains a need for a poly-axial bone screw assembly in which the rod receiving head can be maintained in a desired angular orientation while the rod is positioned within the receiver head.